Similarly to the field lens of a telescope, the main function of the condenser optics in a projection system is to collect as much light as possible from the source and transmit it to a microdisplay or other imaging device, which will spatially modulate the light while transmitting the light towards the projection optics.
The importance of condenser lenses is the fact that the main bottleneck that limits efficiency in commercially available projection systems is their collector optics. Efficiency is a key parameter of projector performance, since it augments screen luminance, enabling the system to perform well under increasing levels of ambient light. Additionally, a higher efficiency also means less heat to extract and so less noise from the fans.
Most conventional condensers use elliptic or parabolic mirrors. They perform very far from the theoretical limits (calculated using the étendue invariance of nonimaging optics) for sources such as arc lamps or halogen bulbs. Typical small displays in the 5-15 mm2·srad étendue range have ray collection efficiencies about 40-50% for the best condensers, although theory allows about 100%.
To understand the limitations of conventional elliptic or parabolic condensers it is useful to consider the concept of the projection of source-images. A pinhole at the exit optical surface of the condenser will pass a bundle of rays that bears the local image of the source (see FIG. 1). A conventional elliptical condenser has a limited collection efficiency for small étendues because of these characteristics of its projected arc images. Most of the drawings show the arc as a surface-emitting cylinder, which is a schematic simplification of the actual non-homogeneous luminance distribution of an arc. Examples of the actual luminance distribution are shown in FIG. 1B and FIG. 23. This simplification is not used in the final design but it helps understand the inherent limitations of the elliptic condenser, because the étendue of the simplified arc is a better defined parameter than that of the real arc.
FIG. 1 shows notable characteristics of the source images of any conventional elliptical condenser: the length of the image is variable depending on which part of the condenser it is imaged by (see FIG. 1A); because they replicate the elongated shape of a light source such as an Ultra High Performance mercury arc lamp (UHP), the images are about 4 times longer than wide (see FIG. 1B); they rotate at the target due to the condenser's rotational symmetry (see FIG. 1C); and the source images do not fit well with the panoramic 16:9 target, shown as a dashed-line rectangle in FIG. 1D.
When the étendue of a microdisplay is much greater than the that of the arc, two of the aspects mentioned above (projected image size variation and images rotation) do not limit the collection efficiency, since the mixing rod entry aperture (shown as a dashed-line rectangle in FIG. 1D) will be much larger than all the projected arc images. This, however, implies the use of a large and expensive microdisplay. In the most cost-demanding applications the microdisplay étendue will be smaller and closer to that of the arc.
An interesting clarifying case is that in which both microdisplay and arc étendues are equal, because 100% ray-coupling efficiency is theoretically possible (since étendue is an optical invariant). It can be shown that in this equal-étendue case the mixing-rod entry aperture will have the same area as the average projected image. This makes it is clear that the variable size and the rotation of the images of the elliptical condenser do prohibit 100% ray-collection efficiency. A condenser achieving such 100% value would necessarily meet the condition that all projected images are rectangular and exactly match the contour of the mixing-rod entry aperture. This condition is general: it does not depend on the optical elements used in such a hypothetical 100% ray-collecting condenser, or even imply that such a design exists.
A recent trend in improving collection efficiency for small étendues has been to reduce the arc étendue by reducing the gap between the electrodes. Furthermore, some optical designs have being developed to improve the efficiency by:                Reducing the arc étendue via a hemispherical mirror concentric to the arc, which reflects half of the emitted light back to the arc (which is partially absorbent) and through it to increase its luminance. The light from the higher-luminance half-étendue arc is then collected by a conventional elliptical reflector.        Creating a side-by-side image of the arc with a decentered hemispherical mirror, and creating a composite 1:1 image of that via a dual parabolic reflector.        The equalization of the meridian length of the projected arc images, by correcting the elliptical mirror coma, using an aspheric reflector profile and an aspheric lens, but both surfaces still being rotationally symmetric.        
These optical approaches, apart from their complexity and technological challenges, have limited gain capability because their optics are still restricted to being rotationally symmetric.
Other approaches to improve system efficiency include color recapture and color scrolling (which try to recover the ⅔ losses produced by the color filtering in single microdisplay projectors), or polarization-recovery techniques (which try to recover the 50% losses produced by the need of polarized light in LCD and LCoS systems). In these approaches, however, the resulting lamp étendue is accordingly increased (doubled and tripled in polarization and color recovery systems, respectively), further limiting the performance of small microdisplays that use classical condensers.
Other free-form XX condenser devices designed with the SMS 3D method were disclosed in our patent application WO 2007/016363 “Free form lenticular optical elements and their application to condenser and headlamps,” which is incorporated herein by reference in its entirety. The devices shown therein in FIG. 43A to 44C, on the contrary to embodiments described herein, are better adapted to circular targets instead of rectangular ones. Those devices use the concept of étendue squeezing, which instead of focusing on avoiding the rotation of projected arc images, produce an equalization of their width and length as indicated in FIG. 43A to 44C of our application WO 2007/016363). The device shown in FIG. 45A-45C of our application WO 2007/016363 has in common with the embodiments in this application that also two halves are made and prevent rotation of images, but the configuration is such that the two halves of the secondary mirror are not adjacent but face one to another.
Regarding the state of the art of manufacturing condenser optics, all present systems are based exclusively on rotational-symmetric surfaces. These are manufactured mainly from glass (due to its low cost) or glass-ceramic (for higher thermal stability). The accuracy of both techniques is limited, so the manufactured profiles can differ substantially from those intended.